Antibradycardia stimulation involves the delivery of controlled stimulation pulses to an atrium and/or a ventricle, using a single or dual chamber device. In the case of cardiac resynchronization therapy (“CRT”), a multisite device delivers the stimulation pulses jointly to both ventricles. In general, after stimulating a cardiac cavity, a test known as a “capture test” is performed to determine whether the stimulation induced a depolarization wave in the cavity (also referred to as an “evoked wave”). A capture test is particularly useful for adjusting the amplitude and/or the width of the stimulation pulses, or the energy delivered to the stimulation site.
There are many techniques for implementing a capture test. For example, a capture test described in WO 93/02741 A1 and its counterpart U.S. Pat. No. 5,411,533 (Sorin CRM, previously known as ELA Medical) uses an automated testing algorithm that measures the efficiency threshold of the stimulation referred to as a “pacing threshold”, at regular intervals, for example, every six hours. The amplitude of the stimulation pulse is then adjusted, based on the measured pacing threshold increased by a safety margin taking into account the various uncertainties in determining the pacing threshold.
It has been discovered that this capture test algorithm in known devices can be misled by some atypical situations, such as an occurrence of a fusion event, in which a stimulation is triggered concomitantly by a spontaneous QRS event, at the time the capture test is performed.
Various proposals have been made to overcome this difficulty, including EP 1216722 A1 and its counterpart U.S. Pat. No. 6,711,441 (Sorin CRM, previously known as ELA Medical), which describes detecting a suspected fusion event and disregarding suspected events in the capture test.
Nevertheless, clinical follow-ups of patients show that the different conventional techniques for performing a capture test remain sensitive to various rhythm abnormalities occurring erratically, which deceive the test algorithm and lead to both false positive and false negative results. These anomalies may lead to an incorrect adjustment of the stimulation energy. If the stimulation energy is set too high, more power is consumed than is needed, thus the lifetime of the implant is reduced. If the stimulation energy is set too low, it presents a potential risk to the patient.
It also is known, as disclosed in EP 1287849 A1 and its counterpart U.S. Pat. No. 6,714,820 (Sorin CRM, previously known as ELA Medical), to perform a capture test, and readjust the stimulation energy continuously by checking on each cycle whether the stimulation was effective. These “cycle-to-cycle” adjustment techniques, despite their much higher reactivity, are very sensitive to the occurrence of a fusion event or an isolated atypical cycle, such as a post-atrial ventricular detection, a too short cycle, or an extrasystole, which may be misinterpreted as a loss of a capture, even though the pacing threshold did not naturally increase.
Moreover, in case of a multisite device, it is necessary to run as many capture tests as there are existing test sites. With the recent trend of increasing the number of stimulation sites, this leads to a substantial increase in a test time needed to perform a capture test on all the stimulation sites.